Let $(E, \langle \cdot, \cdot \rangle)$ be a reproducing kernel Hilbert space on an interval $I$, with reproducing kernel $K$. For all $(x,y) \in I^2$, we set $k_x(y) = K(x,y)$.\\
Let $x \in I$ and $V_x$ defined on $E$ by $V_x(f) = f(x)$. We set
$$N(V_x) = \sup_{\|f\|=1} |f(x)|$$
Prove that
$$N(V_x) = \sqrt{\langle k_x, k_x \rangle}.$$